The present invention relates to an optical communication apparatus using intensity-modulation and, particularly, to an optical communication apparatus in which a received signal can be easily equalized on a receiver side.
In an optical communication system, an intensity modulation-direct detection communication apparatus, in which an intensity modulated signal obtained by modulating an injection current to a semiconductor laser with a signal source is transmitted through a transmission line, an optical fiber, and received by an optical receiver using an opto-electric conversion element such as PIN diode, etc., has been used mainly. In such communication apparatus, it has been known that, when a communication is performed in the low-loss region around a wavelength of 1.5 .mu.m in which optical fiber loss becomes a minimum, a considerable signal quality degradation occurs during propagation through an optical fiber due to chromatic dispersion at a transmission rate of a G bit/s or higher (cf. M. Shikada et al. "Long-distance Gigabit-Range Optical Fiber Transmission Experiments Employing DFB-LD's and InGaAs-APD's", IEEE, Journal of Lightwave Technology, Vol. LT-5, No. 10, October 1987, pp. 1488-1497). Therefore, a transmission using an external modulator has been considered recently to reduce an amount of spectral broadening of a modulated signal light (cf. S. K. Korotky et al. "8-Gb/s Transmission Experiment over 68 km of Optical Fiber using a Ti:LiNbO.sub.3 External Modulator", IEEE, Journal Lightwave Technology, Vol. LT-5, No. 10, October 1987, pp. 1505-1509).
In an optical transmitter using such an external modulator, a band width for signal transmission is very small as compared with that of direct modulation. Therefore, it becomes easy to realize a high density wavelength multiplexing or high density frequency multiplexing. Thus, a further development of this technique is expected.
Further, an optical amplifier has been developed, and a non-repeated cascaded amplifier system using such an optical amplifier is being studied (cf. S. Yamamoto et al. "516 km 2.4 Gb/s Optical Fiber Transmission Experiment using 10 Semiconductor Laser Amplifiers and Measurement of Jitter Accumulation", 17th Conference on Integrated Optics and Optical Fiber Communication, Post-deadline Papers 20 PDA-9). In such non-repeated cascaded amplifier systems, in which a span length can be extended by compensating for signal loss, there is a possibility of ultra long distance transmission. Further, in such ultra long distance transmission, distortion of receiving signal waveforms due to optical fiber dispersion is a major factor limiting the distance over which such a transmission is possible.
In a micro wave communication, such waveform distortion occuring after transmission, in which a signal is directly detected by field intensity and frequency of a carrier can be equalized by using a suitable filter after reception.
In the optical communication system, however, an envelope of optical electric field intensity is squared on a receiving side, and only an intensity component thereof is received as an electric signal, and frequency and phase components of the optical electric field are missing. On the other hand, it is known that, due to a chromatic dispersion characteristic of a transmission path, distortion to be applied to the signal is different on an upper side-band and a lower side-band, but these frequency and phase components are missing in the optical communication. Therefore, the distortion characteristics of an electric signal are so complicated that it becomes very difficult to be compensated by a conventional equalizer.
This will be described in more detail below.
An electric field E(t) of light obtained by intensity-modulating laser light with transmission signal a(t) is represented by: EQU E(t)=[1+a(t)] cos 2.pi.ft (1)
f: carrier frequency PA1 f.sub.o : transmission rate of transmission signal
A Fourier expansion of the transmission signal a(t) becomes as follows: ##EQU1## where, n: integer
The equation (1) becomes: ##EQU2##
In general, in the direct detection, a light intensity obtained by squaring light electric field is received. An intensity component I.sub.DB (t) of this signal can be calculated as follows: ##EQU3## where a.sub.n '&lt;1 and a.sub.n .multidot.a.sub.n '&lt;1 are assumed.
Now, it is assumed that such signal represented by the equation (2) propagates along a transmission path such as optical fiber having wavelength dispersion. The electric field E(t) of light after transmission is represented as follows: ##EQU4## The intensity component I.sub.DA (t) obtained by square-law detection of the field E(t) is as follows: ##EQU5## The phase delays .PSI.(f+nf.sub.o), .PSI.(f-nf.sub.o) of the equation (5) are sources of waveform distortion after transmission.
An equalization of the intensity-modulated component received to the intensity signal of an original signal before transmission will be considered next. It is clear from the equation (5) that the phase delay of the component .PSI.(f+nf.sub.o) having the frequency nf.sub.o in the second term and third term is reversed in direction with respect to that of the component -.PSI.(f-nf.sub.o) having the same frequency in the fourth and fifth term. This means that, in I.sub.DA (t), the phase delay distortion .PSI.(f-nf.sub.o) occurred in the lower side-band is coverted to a phase advance distortion -.PSI.(f-nf.sub.o) in the received intensity-modulated component and that the phase delay distortion .PSI.(f+nf.sub.o) occurred in the upper side-band appears in I.sub.DA (t) as it is. That is, in order to equalize the received signal of the equation (5), it is necessary to simultaneously equalize the phase delay distortion .PSI.(f+nf.sub.o) and the phase advance distortion -.PSI.(f-nf.sub.o). This necessity causes an equalization of the received signal in the equation (5) by means of a conventional linear filter to be very difficult.
In order to solve this problem, Gnauck et al. have proposed a system in which dispersion is compensated by inserting a optical filter suitable in a light signal frequency range into a front portion of a receiver (cf. A. H. Gnauck et al. "Optical Equalization of Fiber Chromatic Dispersion in 5-Gb/s Transmission System", 1990 Optical Fiber Communication Conference, Post deadline paper, PD7"). However, this system is still not effectively used since it is very difficult to make the characteristics of an optical filter equal to a reversal of the dispersion characteristics of a transmission path.